The Army private arrested in the biggest leak of classified material in US history has pleaded guilty to 10 charges that could send him to prison for 20 years, saying he was trying to expose the American military's "bloodlust" and disregard for human life in Iraq and Afghanistan.

Military prosecutors said they plan to move forward with a court-martial on the 12 remaining charges against Bradley Manning, including aiding the enemy, which carries a potential life sentence.

For the first time, Manning directly admitted leaking the material to the anti-secrecy website WikiLeaks and detailed the frustrations that led him to do it.

"I began to become depressed at the situation we found ourselves mired in year after year. In attempting counter-insurgency operations, we became obsessed with capturing and killing human targets on lists," the 25-year-old former intelligence analyst in Baghdad told a military judge.

He added: "I wanted the public to know that not everyone living in Iraq were targets to be neutralised."

The slightly built soldier read from a 35-page statement through his wire-rimmed glasses for more than an hour. He spoke quickly and evenly, showing little emotion even when he described how troubled he was by what he had seen.

The judge, Col Denise Lind, accepted his plea to 10 charges involving illegal possession or distribution of classified material. Manning was allowed to plead guilty under military regulations instead of federal espionage law, which knocked the potential sentence down from 92 years.

He will not be sentenced until his court-martial on the other charges is over.

Manning admitted sending hundreds of thousands of Iraq and Afghanistan battlefield reports, State Department diplomatic cables, other classified records and two battlefield video clips to WikiLeaks in 2009 and 2010.

He said he was disturbed by the conduct of the wars in Afghanistan and Iraq and the way American troops treated the populace. He said he did not believe the release of the information would harm the US.